System for generating instant contours from stereo pairs of aerial photographs

ABSTRACT

A system for generating instant contours from stereo pairs of aerial photographs. The optical system operates on all information on both aerial photographs simultaneously and in parallel instead of using a scanning technique as is commonly done.

KR 3961339532 r Inventors Appl. No.

Filed Patented Assignee Nicholas K. Sheridon Fairport, N.Y.;

Daniel C. Kowalski, Southgate, Mich. 848,423

Aug. 8, 1969 Sept. 7, 1971 The United States of America as representedby the Secretary of the United States Air Force SYSTEM FOR GENERATINGINSTANT CONTOURS FROM STEREO PAIRS 0F AERIAL PHOTOGRAPHS 5 Claims, 7Drawing Figs.

US. Cl 356/2, 350/35 lnt.Cl "GORE/0 4, (3172b Field oISearch 356/2,7l;

[56] Retereuces Cited UNITED STATES PATENTS 3,552,858 l/l97l Haines etal. 35613.5 X OTHER REFERENCES Haines et al.: Contour Generation byWavefront Reconstruction Physics Letter, Vol. 19, No. 1, Sept. 15, 1965,pages 10 and 11 Hildebrand et al.: The Generation of Three-DimensionalContour Maps by Wavefront Reconstruction, Physics Letters, Vol. 21,No.4, June I, 1966 pg. 422, 423

Primary Examiner-Ronald L. Wibert Assistant Examiner-F. L. EvansAttorneys-Harry A. Herbert, Jr. and Jacob N. Erlich ABSTRACT: A systemfor generating instant contours from stereo pairs of aerial photographs.The optical system operates on all information on both aerialphotographs simultaneously and in parallel instead of using a scanningtechnique as is commonly done.

his no mal? SYSTEM FOR GENERATING INSTANT CONTOURS FROM STEREO PAIRS OFAERIAL PHOTOGRAPHS BACKGROUND OF THE INVENTION This invention relatesgenerally to an optical system, and more particularly to a series ofcoherent optical systems that will perform instant contouring fromstereo pairs of serial photographs.

A major source of military intelligence information is airphotointerpretation. Analysis of terrain elements gives data on observationpoints, cover and concealment, trafficability, routes for movement ofmen and equipment and obstacles thereto, suitability for various typesof military installations, and characteristics of beaches which affectlanding operations. Tactical interpretation aids in camouflage detectionand location and appraisal of fortifications, gun emplacements,airfields, supply depots, naval bases, and other military factors.

Generally such photos are made along parallel flight lines, overlappingalong and between lines so as to give a continuous composite picture ofthe earths surface. Methods are in use for measuring of heights, slopeinclinations, distances, and directions directly from photos, and formaking planimetric and contour maps. Through airphoto interpretationutilizing the three-dimensional view provided by overlapping photosviewed under the stereoscope, qualified specialists can obtain a widerange of scientific and engineering information on natural and culturalfeatures of the earths surface.

SUMMARY OF THE INVENTION The instant invention sets forth severalsystems of generating contours from stereo pairs of aerial photographs.The isoaltitude contours or contour intervals may be generated orseparated from the surrounding imagery by performing a correlationprocess between the conjugate pair of stereo photographs. The lightresulting from this correlation process is separated from noncorrelationlight or treated in such a way to identify it with respect to thenoncorrelation light. By the techniques of the instant invention setforth in detail hereinbelow light may be reimaged to form contours or inturn may be used to reimage the image data from which contour intervalswere made.

By instant contouring" it is meant that the optical system of thisinvention operates on all the information on both photographssimultaneously and in parallel instead of using a scanning technique asis commonly done. The inherent advantages of coherent optical systemsfor doing instant contouring are principally speed and accuracyresolution.

Isoaltitude contours can be produced by the present systems describedhereinbelow in a number of forms. These contours may be dark linessuperimposed on imagery or standing alone. They may be bright linessuperimposed on the imagery or produced separately. Some systems willeven extract separate from the rest of the photographic image thatimagery which lies within a contour interval. Another advantage of thecoherent optical system of this invention is the manner in whichisoaltitude information can be extracted from the images. It will beseen that among the principal differences among the contour generatingsystems described is the ability to minimize the noise introduced duringthe contouring process.

The imagery may be introduced in two forms in many of the opticalsystems set forth below. The imagery may be introduced as amplitudemodulation which is the case of an ordinary photograph. It may also beintroduced as a pure phase function. This is generally obtained bybleaching an ordinary photograph. In the bleaching process, the emulsionis shrunken in proportion to the intensity of light that originallyexposed the photograph. There is essentially no information lost in thisprocess. It will be seen, however, that the use of bleached photographswill, in some cases, tend to minimize the noise introduced in thecontouring system. It is expected that when bleached functions are usedcare will be required in making certain that corresponding bits ofimagery on the two stereo photographs are processed so as to provide thesame depth of modulation. That is, the shrinkage of the two photographsfor a given piece of information will be essentially the same.

It is therefore an object of this invention to provide an instantcontouring system which is accomplished with great speed and yet ishighly accurate.

It is another object of this invention to provide a coherent opticalsystem in which isoaltitude information can be extracted from images.

It is still another object of this invention to provide a contourgenerating system which produces a minimum of noise during thecontouring process.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription taken in connection with the accompanying drawing.

DESCRIPTION OF THE DRAWING FIG. 1 represents a schematic view of thefirst instant contouring system of this invention;

FIG. 2 represents a schematic view. of the second instant contouringsystem of this invention;

FIG. 3 represents a schematic view of the third instant contouringsystem of this invention;

FIG. 4 represents a schematic view of the fourth instant contouringsystem of this invention;

FIG. 5 represents a schematic view of the fifth instant contouringsystem of this invention;

FIG. 6 represents a schematic view of the sixth instant contouringsystem of this invention; and

FIG. 7 represents a schematic view of the seventh instant contouringsystem of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First System The schematicdiagram of the first system 10 is shown in FIG. I. A monochromatic,coherent, light source is positioned at point 12 and one of the stereophotographs is placed in plane 14. A pair of spherical lenses l6 and 18are placed at either side of plane 14. The stereo photographs may be ableached function or-on amplitude function. A Fourier transform of thisphotograph forms in plane 20 where a high-pass filter 22 removes thedirect current or DC term and the low spatial frequencies that providepoor correlation discrimination. The photograph is again imaged withoutthe DC term through spherical lens 24 in plane 26, where it is recordedas a hologram by means of monochromatic reference beam 28 throughspherical lens 30. The second stereo photograph is now placed in plane14. The hologram in plane 26 has recorded both the first stereophotograph and its complex conjugate. The imagery of the second photowill now be multiplied times the complex conjugate of the imagery of thefirst photograph in plane 26. Those regions that correlate, namely allthe information in a given isoaltitude contour interval, will form theplane correlation wave. This will have the shape of the contourinterval. If the input functions have not been bleached, this will be anamplitude modulated plane correlation wave. A transform of this wave istaken through spherical lens 32 in plane 34 where a low-pass filter 36removes all but the zero spatial frequency component of the correlationfunction. In plane 38 will be imaged those regions of the imagery inplane 26 that have been correlated. This light distribution is a singleisoaltitude interval. It contains no image information per se. If thefunctions had not been bleached, this isoaltitude interval will becharacterized by a random distribution of low spatial frequency noise.If bleached, this noise will not be present. A further advantage of thebleaching process occurs when the functions are bleached to a very largedepth of modulation. This will destroy the correlation process in allbut those regions which overlie very exactly. This will give rise toacontour image in plane 38 of a very harrow altitude interval. If thesecond stereo photograph is now placed in plane 38, the imagery in thisisoaltitude contour interval alone will be illuminated. Otherisoaltitude contour intervals may be obtained by moving the secondaerial photograph along the flight line.

Second System The second system 40 is a variation of system with certainadvantages and is shown in FIG. 2. With system 40 the contours can bepresented in three forms.

To operate the second system 40, a monochromatic, coherent light sourceis positioned at 42, the first of the stereo pair of aerial photographsis placed in plane 44. Again as in FIG. 1 a pair of spherical lenses 46and 48 are placed at either side of plane 44. The Fourier transform istaken in plane 50 where low spatial frequencies that provide poorcorrelation are removed by filter S2. The filtered photograph is imagedthrough spherical lens 54 in plane 56 where, by means of a monochromaticreference beam originating at point 58 through lens spherical 60 ahologram is made. The stereo photographs in this case may or may not bebleached. If one is bleached, the other one must also be bleached.Stereo photograph No. 2 is now placed in plane 44 and its image in plane56 will multiply against the complex conjugate of the image of aerialphotograph No. 1 as recorded on the hologram. This gives rise to a planecorrelation wave moving in the reference beam direction for thoseportions of the two photographs that are lined up correctly. As before,these portions define an isoaltitude contour interval. We may treat thiscross-correlation light in three ways now.

In the first of these spherical lens 61 is positioned after plane 56 anda low-passfilter 62 is placed in plane 64. The filtered light isreimaged through lens spherical 66 in plane 68 where by means of amonochromatic reference beam originating at 70 and passing throughspherical lens 72 is recorded as a hologram. The isoaltitude contourinterval will now be a bright patch of light in plane 68. As before, ifthe functions were not phase functions, there will be predominantlowfrequency noise across the light distribution. This is onepresentation of the contours. To obtain another presentation of thecontours, monochromatic reference beam originating at 74 is turned on inplace of the beam at 70. The beam at 74 moves in the opposite directionto the beam at 70 and passing through spherical lens 76 has the effectof creating the conjugate image moving back along the optical system.This is identical to the light which originally exposed the hologramexcept for the direction of propagation. The low spatial frequencyfilter 62 in plane 64 can now be removed (shown in phantom). Thecorrelation light will now illuminate the hologram in plane 56. Theimagery in the isoaltitude contour interval being treated will now beprojected in plane 44. All other imagery will be heavily suppressed orabsent. In this manner the contour interval is presented explicitly interms of the imagery contained within it.

A second variation of this involves placing a very small halfwave plate78 of diameter equal to the diameter of the lowpass filter 62 that mightordinarily be placed in plane 64 instead of filter 62. Thus, all thelight moving in the reference beam direction, whether correlation lightor not, is recorded in plane 68 as a hologram. The half-wave plate 78 isnow removed from plane 64 and by means of reference beam from point 74the imagery is reprojected down the axis. Assume now that the stereoaerial photographs have been bleached so they are pure phase functions.The product of this reprojected light with the hologram in plane 56 willgive rise to an image of stereo photo No. 2 reprojected down thehorizontal optical axis. This will reform in plane 44 as expected.However, all imagery in the isoaltitude contour interval beingconsidered will now be 180 out of phase with the surrounding imagery. Inregions of overlap, there will be a narrow, dark interference bandsurrounding the isoaltitude contour interval. Thus, the imagery may bephotographed intact in plane 4 with contour lines on it.

Variation number three in this contouring concept also takes placeduring the initial correlation process. Once again in place of thelow-pass filter 62 in lane 64 a very small direct current or DC block 80essentially a high-pass filter, is placed there. In plane 68 will form arandom noise background with dark regions corresponding to theisoaltitude contour intervals superimposed on it. This may besuperimposed on the imagery by recording the light in plane 68 as ahologram by means of reference beam originating at 70. Once againreference beam from 74 is used to reproject this light down the inclinedoptic axis. This light multiplies against the hologram in plane 56 andin plane 44 will form an image of aerial photograph 2. All informationin the isoaltitude contour interval has now been removed, however, andwill appear as a dark band against the imagery off aerial photo 2. Otherisoaltitude contour intervals may be obtained by moving aerialphotograph 2 along the flight line.

This system, therefore, will produce isoaltitude contours as brightbands, as dark bands, as dark bands superimposed upon imagery, as theimagery contained in the interval alone, or as dark bands surroundingthe imagery in the interval and superimposed upon the surroundingimagery. This system has a disadvantage operationally. It requires usingthe light from a hologram to reconstruct a hologram. This means thatlight available for the final imaging step is very low in intensity. Itwill be seen that system three (shown below) circumvents thisdifficulty. This difficulty is not considered to be a fundamentallygreat disadvantage however.

Third System The third system 82 shown in FIG. 3 is very similar to thesecond system 40. It has the advantages of more efficient handling andthe ability to produce multiple contours simultaneously. Its basis is arecently developed correlation concept.

It is assumed that the aerial photographs used in this concept arebleached functions. This is not essential but it is felt the resultswill be much better in this case.

A monochromatic, coherent light source 84 passes its beam throughspherical lens 86. An aerial photograph No. l is placed in plane 88.Another lens is located after plane 88. The Fourier transform ofphotograph No. l is taken in plane 94 where a low-pass filter 96 removesthose spatial frequencies which provide poor correlation. Thisphotograph is reimaged through spherical lens 98 in plane 100 where itis bleached and made a pure phase function. The photograph is removedfrom plane 88 and the Fourier transform of the bleached photograph istaken in plane 102 after passing through spherical lens 101 whereanother low-pass filter 104 removes the DC component. The image isformed in plane 106 after passing through spherical lens and the Fresneldiffraction pattern in plane 108 where a hologram is made by the aid ofa monochromatic reference beam originating at 110 and passing throughspherical lens 112. The second aerial photograph is now placed in plane88 and a bleached copy is obtained in plane 100 as before, and thelow-pass filter 96 is removed from plane 2 (shown in phantom). By meansof the monochromatic reference beam. from 114 through lens 116 theFresnel diffraction pattern in plane 108 is now reconstructed movingdown the optic axis in the opposite direction from which it originallycame. This is identical to the original Fresnel diffraction pattern inevery respect except the direction of propagation. What is effectively acomplex conjugate image of aerial photograph l is now formed in plane100 where it is multiplied against the bleached version of aerialphotograph 2. Those portions of the two photographs in correlation,namely, the isoaltitude contour interval in question, will produce aplane unmodulated wave front. This correlation light is separated fromnoncorrelation light in plane 94 by means of low-pass filter 96 andimaged in plane l. The contour interval is now a bright band of light ofthe correct shape. The amplitude version of aerial photograph 2 can nowbe rein serted in plane 88. Only that imagery in the isoaltitude contourinterval will now be illuminated. This is one presentation of thecontour information. A second presentation may as before be obtained byputting a half-wave plate 118 of small dimensions in plane 94 in placeof filter 96. Now all the imagery on aerial photograph 2 will beilluminated. That which illuminates the information in the isoaltitudecontour interval, however, will be l80 out of phase with the surroundinglight, and this imagery will therefore be surrounded by a darkinterference band which defines the interval. The third variation ofthis scheme involves placing a small DC block or high-pass filter 120 inplane 94 in place of filter 96. Now, all the imagery of aerialphotograph 2 will be illuminated with the exception of the imagerycontained in the isoaltitude contour under consideration. Otherisoaltitude contour intervals may be obtained by moving aerialphotograph 2 along the flight line.

In addition to the facility with which the various isoaltitude contoursmay be presented with this scheme, there is a possibility of presentingmore than one isoaltitude contour at a time. This is done by placing adiffuse light source 111 (shown in phantom) in the back focal plane oflens 112 in reference beam from 110, or alternately, in placing severalpoint sources in the back focal plane of lens 112. Each point of lightin this new light source will now produce a reference beam which willreconstruct the entire Fresnel diffraction pattern of aerial photograph1 recorded in plane 108. Each of these Fresnel diffraction patterns willbe moving, however, in a somewhat different direction so that in plane100 we will have a number of displaced images of aerial photograph lsuperimposed or multiplied against aerial photograph 2 in that plane.Different regions of imagery will now correlate. For each of thesecomplete images a contour interval will be defined. Hence, a number ofcontouring intervals may be generated simultaneously. Obviously, inplace of single low-pass filter 96 or the single DC block 120, or thehalf wave plate 118 in plane 94 we will now have as many of these as wehave contour intervals we wish to generate.

We have the option with this presentation of multiple contours of onlyone way of presenting the contour information, however. A series oflow-pass filters 122 must be placed in plane 94 whose separation andsize will permit correlation light from only one isoaltitude contourinterval to pass through each hole. The imagery placed in plane 88 willnow be illuminated at only those positions where the isoaltitudecontours have been found.

FOURTH SYSTEM The fourth system 124 is based upon the so-calledimageimage correlator. This system is shown in FIG. 4. A monochromatic,coherent light source 126 is located before spherical lens 128. Aerialphotograph l is placed in plane 130. Its Fourier transform is producedafter passing through spherical lens 132 in plane 134 where a high-passfilter 136 will remove those spatial frequencies which provide poorcorrelation. This is reimaged after passing spherical lens 138 in plane140. Also, in plane 140 is placed aerial photograph 2. In those regionsof the two photographs which correlate (determined by the alignments ofthe aerial photographs along the flight line) a plane correlation wavewill be generated. This will be focused after passing spherical lens 142to a point in plane 144. The correlation light will be separated fromthe surrounding light by means of a low-pass filter 146 in plane 144 andreimaged after passing spherical lens 148 in plane 150. Photograph 2 maybe placed in plane 151); the correlation light will illuminate all theimagery in the isoaltitude contour under consideration. The inputfunctions may be bleached or unbleached. It is felt that there will beless extraneous noise generation if the input functions are bleached.However, it is also felt that in this particular configuration the depthof modulation of the bleaching process should not be too great.

A very distinct advantage of this correlation scheme is that it does notrequire the time delay involved in making a hologram.

FIFTH SYSTEM System 152 is a variation of the fourth system 124 modifiedto provide a greater precision and versatility of contour intervalpresentation. This is shown clearly in FIG. 5.

To be an advantage, the aerial photographs used in this concept must bebleached.

As before a monochromatic, coherent light source 154 passes its beamthrough spherical lens 156. Aerial photograph l is placed in plane 158.Its Fourier transform after passing spherical lens is taken in plane162, low spatial frequencies are removed by filter 164 and after passingspherical lens 166 is reimaged in plane 168. Aerial photograph 2 is nowplaced in plane 168. We now have the option of the three variations thatwe had in system 40 shown in FIG. 2. The first variation is a low-passfilter 170 placed after lens 169 in plane 172. This passes thecross-correlation light, namely, light from a given isoaltitude contourinterval, and after passing spherical lens 174 in plane 176 a brightband of light corresponding to this interval will form. This is recordedas a hologram by means of a monochromatic reference beam originating at178 and passing spherical lens 180. This light may be reconstructedmoving in the opposite direction down the axis by means of amonochromatic reference beam originating at 182 passing spherical lens184. In plane 158 only that imagery that was present in the giveniosaltitude contour interval will be illuminated and may be recorded bya photographic plate in that position. Variation No. 2 involves placinga very small diameter half wave plate 186 in place of filter 170 inplane 172 during the correlation process. The light is now recorded inplane 176 by means of monochromatic reference beam originating at 178.By means of reference beam from 182 all the imagery in stereo photographNo. I recovered in plane 158. That imagery contained in the givenisoaltitude contour, however, will be out of phase with the surroundingimagery and therefore, as before, this imagery will be surrounded by adark interference band. Variation 3 involves replacing filter 170 withhigh-pass filter 188 in plane 172. Correlation light is now recorded inplane 176 by means of reference beam from 178 and by means of referencebeam from 182 this light is passed again down the axis. In plane 158,all imagery contained in the given isoaltitude contour interval willappear as a dark band against the surrounding imagery. Motion ofphotograph 2 along the flight line will produce other isoaltitudecontour intervals.

SIXTH SYSTEM This system 190 makes use of the so-called matched-filtercorrelator. Like system 82 shown in FIG. 3 this scheme enables one toderive a number of contour intervals simultaneously.

The schematic drawing for this instant concept contouring is shown inFIG. 6. Aerial photograph l is placed in plane 192 after monochromatic,coherent light source 194 and spherical lens 193. (Amplitude modulatingphotographs are used). The Fourier transform is taken after passingspherical lens 195 in plane 196 to remove undersired low spatialfrequencies by filter 19S and it is reimaged through spherical lens 200in plane 202. After passing spherical lens 204 another Fourier transformis taken in plane 206 where it is recorded by means of a monochromaticreference beam originating at 208 and passing spherical lens 210 as ahologram, (or, in more common terms, a matched filter). Aerialphotograph No. 2 is now placed in plane 192 and its Fourier transformforms in plane 206. The Fourier transform of aerial photograph 2 willnow multiply against the complex conjugate of the Fourier transform ofphotograph l which is recorded on the hologram. Those portions of thetwo photographs which correlate will produce plane correlation wavefronts moving in the general direction of the reference beam originatingat 208. Each isocontour interval will produce a plane wave front movingin a slightly different direction. The angle will be proportionate tothe displacement of the isocontour intervals between the twophotographs. Each will, therefore, come to a focus in a differentposition after passing spherical lens 212 in plane 214. We now have theoption of following any one of three possible variations in thiscontouring scheme, depending upon the type of contour presentation wewish to obtain.

Variation I involves placing a series of low-pass filters 216 in plane214. This is in essence a series of srnall holes on an opaque support.Each hole will pass in isoaltitude contour interval and as manyintervals may be obtained as there are holes in plane 214. Correlationlight passed through the holes on plane 214 is now imaged throughspherical lens 218 on plane 220 and a hologram formed from it by meansof monochromatic reference beam originating at 222 and passing sphericallens 224. The imagery in plane 220 is now a series of bright bands, eachcorresponding to a given isoaltitude contour interval. By means ofmonochromatic reference beam originating at 226 and passing sphericallens 228 the light information recorded in plane 220 may be reprojectedback along the inclined optic axis. This is identical to the light whichoriginally came up this optic axis (above plane 214), except for itsdirection of propagation. When this light is multiplied against thishologram in plane 206 the image will be reconstructed in plane 192. Thatis, the imagery contained in the isoaltitude contour intervals passed bythe low spatial frequency filters 216 in plane 214 will appear in plane192. Imagery not contained within these intervals will be presented asdark bands. Variation 2 in this concept involves placing a series ofhalf wave plate 230 in place of filters 216 in plane 214 identical insize and position to the low-pass filters 216 in variation 1. Hence, allthe correlation light will be passed on to plane 230. When this light isreformed by reference beam from 226 and the input image 2 is recoveredin plane 192, the entire image will be present. However, thatinformation contained in the isocontour intervals in question will be180 out of phase with the surrounding imagery. Therefore, at thejunctionof isocontour intervals there will be a dark interference band, whichwill constitute our contour information. Variation 3 involves placing inplane 214 a series of high-pass filters 232 in place of filters 216comprising of small DC blocks of size and position identical to the halfwave plates 230 used in variation 2. We will then obtain in plane 192once again the entire image of aerial photograph 2 with the exception ofthe imagery contained in the isocontour intervals which will bepresented as dark bands.

As pointed out previously, one of the major advantages of system 190 isthat, like system 82, it permits presentation of a number of isocontourintervals simultaneously. Unlike system 82, however, it permitspresentation of more intervals before the introduction of noise becomesprohibitive. However, a drawback to system 190 lies in the fact that thematched filter recorded in plane 206 will contain both amplitude andphase information. This will amplitude modulate the correlation lightwith noise. This amplitude information may be eliminated by an inversepower spectrum filter 234 (shown in phantom) during the correlationprocess. That is, by a filter which records the power spectrum of theFourier transform present in plane 206 in the denominator. Such a filtercan be constructed by conventional hologram techniques using a referencebeam which is exactly in line with the optic axis of the system.However, this will eliminate only the amplitude information on thematched filter due to the Fourier transform of aerial photograph l.

SEVENTH SYSTEM This system 236 shown in FIG. 7 is an interferometricapproach. its main advantage lies in the ability to view all theisocontour intervals at a given time as a virtual image.

System 236 requires the use of bleached functions. Again amonochromatic, coherent light source 238 is positioned before sphericallens 240. A bleached version of aerial photograph l is placed in plane242 in H6. 7. The Fourier transform is taken in plane 244 where the DCterm is removed by filter 246 after passing spherical lens 245. It isprobably not necessary to remove low spatial frequencies other than theDC term. This function is reimaged in plane 248 after passing sphericallens 250 where it is recorded as a hologram by means of the indicatedmonochromatic reference beam originating at 252 and passing throughspherical lens 254. it should be noted that plane 248 need not be animage plane; it could equally well be a Fresnel diffraction plane.Aerial photographiZ is now placed in plane 242. This is a bleachedfunction also, but it has some amplitude modulation. The indicatedreference beam is turned on to reconstruct an image of aerial photographl in plane 256 after passing lens 258 su' perimposed upon that of aerialphotograph 2. if the phase in the illumination light or the referencebeam light is now changed by A2 dark interference bands will appear inplane 256 wherever the images are identical. it should be noted at thispoint that this technique requires rather precise control over thebleaching process so that both photographs have the same depth ofmodulation. The interference fringes appearing in plane 256 will be darkbands superimposed upon the image of aerial photograph 2. Since bothphotographs have been bleached they will superimpose a noise pattern onplane 256.

If a diffuse source is placed for the viewing process in place of thepoint source 238 used to make the hologram in plane 248 it is expectedit will be possible to look along the axis of the system and see inplane 256 a single isoaltitude contour interval superimposed on theimage of aerial photograph 2. If the position of the eye is changed itis expected another isoaltitude contour interval will replace the firstand in this manner by sufficient motion all isoaltitude contourintervals should be visible. These may also be superimposed upon theimage by placing aerial photograph 2 in plane 256.

Although the invention has been described with reference to particularembodiments, it will be understood to those skilled in the art that theinvention is capable of a variety of alternative embodiments within thespirit and scope of the appended claims.

We claim:

1. A system for generating instant contours from stereo pairs ofphotographs comprising a monochromatic coherent light source, a firstspherical lens adjacent said light source, a second spherical lensadjacent to and optically aligned with said first lens for producing ina first plane a Fourier transform of each of a pair of stereophotographs positioned between said first and second lens, a filtermeans located in said first plane for removing the direct current termfrom said transform, a third spherical lens adjacent to and opticallyaligned with said first plane for reimaging each of said photographswithout said direct current term in a second plane, a secondmonochromatic light source located adjacent said third lens, a fourthspherical lens located adjacent said second light source, said secondlight source directing its light through said fourth spherical lensthereby forming a hologram of both said first photograph and its complexconjugate in said second plane, the imagery of said second photographbeing multiplied times said complex conjugate of the imagery of saidfirst photograph in said second plane, whereby those regions whichcorrelate will form a plane correlation wave having the shape of thecontour interval.

2. A system for generating instant contours from stereo pairs ofphotographs comprising a monochromatic coherent light source, a firstspherical lens adjacent said light source, a second spherical lensadjacent to and optically aligned with said first lens for producing ina first plane a Fourier transform of each of a pair of stereophotographs positioned between said first and second lens, a filtermeans located in said first plane for removing the direct current termfrom said transform, and a third spherical lens adjacent to andoptically aligned with said first plane for reimaging each of saidphotographs without said direct current term in a second plane, a fourthspherical lens in optical alignment with and adjacent said second plane,a second monochromatic light source located adjacent said fourth lens, afifth spherical lens located adjacent said second light source, saidsecond light source directing its light through said fifth lens therebyforming a hologram in a third plane located in optical alignment withand adjacent said fourth lens, a Fourier transform of said secondphotograph being multiplied against the complex con jugate of theFourier transform of said first photograph recorded on said hologram,whereby those portions of said two photographs which correlate willproduce plane correlation wave fronts and each isocontour will produce aplane wave front moving in a slightly different direction.

3. A system as defined in claim 2 wherein a low-pass filter is locatedin said third plane.

4. A system as defined in claim 2 wherein a half wave plate

1. A system for generating instant contours from stereo pairs ofphotographs comprising a monochromatic coherent light source, a firstspherical lens adjacent said light source, a second spherical lensadjacent to and optically aligned with said first lens for producing ina first plane a Fourier transform of each of a pair of stereophotographs positioned between said first and second lens, a filtermeans located in said first plane for removing the direct current termfrom said transform, a third spherical lens adjacent to and opticallyaligned with said first plane for reimaging each of said photographswithout said direct current term in a second plane, a secondmonochromatic light source located adjacent said third lens, a fourthspherical lens located adjacent said second light source, said secondlight source directing its light through said fourth spherical lensthereby forming a hologram of both said first photograph and its complexconjugate in said second plane, the imagery of said second photographbeing multiplied times said complex conjugate of the imagery of saidfirst photograph in said second plane, whereby those regions whichcorrelate will form a plane correlation wave having the shape of thecontour interval.
 2. A system for generating instant contours fromstereo pairs of photographs comprising a monochromatic coherent lightsource, a first spherical lens adjacent said light source, a secondspherical lens adjacent to and optically aligned with said first lensfor producing in a first plane a Fourier transform of each of a pair ofstereo photographs positioned between said first and second lens, afilter means located in said first plane for removing the direct currentterm from said transform, and a third spherical lens adjacent to andoptically aligned with said first plane for reimaging each of saidphotographs without said direct current term in a second plane, a fourthspherical lens in optical alignment with and adjacent said second plane,a second monochromatic light source located adjacent said fourth lens, afifth spherical lens located adjacent said second light source, saidsecond light source directing its light through said fifth lens therebyforming a hologram in a third plane located in optical alignment withand adjacent said fourth lens, a Fourier transform of said secondphotograph being multiplied against the complex conjugate of the Fouriertransform of said first photograph recorded on said hologram, wherebythose portions of said two photographs which correlate will produceplane correlation wave fronts and each isocontour will produce a planewave front moving in a slightly different direction.
 3. A system asdefined in claim 2 wherein a low-pass filter is located in said thirdplane.
 4. A system as defined in claim 2 wherein a half wave plate islocated in said third plane.
 5. A system as defined in claim 2 wherein ahigh-pass filter is located in said third plane.